Electron-scale dynamics and generalized Ohm’s law of an MMS X-line encounter on 27 August 2018

Magnetic reconnection is a fundamental process in collisionless space plasma, and the electron-scale kinetic physics at the X line controls how the magnetic field lines break and reconnect. The four spacecraft of the Magnetospheric Multiscale (MMS) mission encountered an X line of symmetric reconnection in the terrestrial magnetotail on 27 August 2018. Here, we present the electron-scale dynamics and the generalized Ohm’s law (GOL) analysis of this case. Its two-dimensional structure, magnetic topology, and electron streamline map are reconstructed based on a time-independent and inertialess form of electron magnetohydrodynamic (eMHD) equation. We map the electron velocity distribution functions (VDFs) along the MMS trajectories through the X line, covering the two-side inflow and reconnected regions, and the typical electron motions for forming the observed VDFs are also presented. The observed reconnection electric field E_M is approximately 2-3 mV/m and predominantly balanced by the spatial gradient of the electron pressure off-diagonal term P_eMN, which is mostly contributed by the electron meandering motion at the X line. Our results show the electron-scale dynamics and the associated electron VDFs at an X line and their role in the electron force balance.